In a typical multimedia playback application, digital audio and digital video signals are created at a source and delivered in a mixed stream or in separate streams over a communications network such as the Internet to an intelligent playback device such as a personal computer equipped with an Internet browser and multimedia playback tools.
As is well known in the art, digital audio signals are typically created as an ordered sequence of digital samples of an analog audio signal. The analog signal is periodically sampled at a fixed interval, the inverse of which is known as the audio sampling rate. For storage or transmission purposes, the sequence of audio samples is often divided into non-overlapping segments known as audio blocks, with each audio block containing a prescribed number of audio samples.
Digital video signals are typically captured and represented as a sequence of still images in digital form. Each video image is generally known as a video frame. The number of video frames captured per unit of time is known as the video frame rate.
For efficiency in transferring video frames and audio blocks to a multimedia player, a multimedia source generally encodes this video and audio information. This information is often encapsulated in data packets (packetized) at the source before transmission over a data network to a player, with information added to the packets indicating the appropriate playback sequence and time for each transmitted audio block and video frame. At the player, encoded information is extracted from the packets, and then either directly decoded and played or stored for later processing. The playback of decoded data through a video or an audio playback device is commonly referred to as "rendering."
Most video encoding techniques, as reflected in standards such as MPEG-I, MPEG-II and H.263, use inter-frame coding to compress or reduce the amount of video information that must be transmitted to the player. Because only a subset of the pixels encoded in each digital video frame typically changes over successive frames, the amount of video information transmitted can be substantially reduced by encoding and transmitting only those pixels that change over successive frames. Such encoding techniques typically require the playback device to successfully decode one or more video frames adjacent to the current video frame in order to fully decode the current frame. In the event that adjacent video frames are ineffectively decoded or lost, most video encoding techniques also periodically insert "key" frames into the video stream, which contain sufficient information to be decoded independently of adjacent frames and can be used to restart the compressed video decoding process when necessary.
Because video frames require substantially more data than audio blocks, the multimedia data stream generally contains a smaller number of audio packets randomly interspersed among a larger number of video packets. These infrequently interspersed audio packets can lead to misalignment in the intended playback sequence of video frames and audio blocks upon arrival at the playback device. A number approaches have been applied to re-synchronize video and audio packets upon arrival (see, e.g., U.S. Pat. No. 5,598,352, issued to Rosenau et al., hereinafter referred to as the Rosenau '352 patent).
Even with re-synchronization of multimedia data stream elements upon arrival at the playback device, end-users may find multimedia playback performance to be inadequate. Although multimedia data may be fully streamed or otherwise stored in sequence on a playback device before playback begins, devices with low processing power or the overhead of other substantial and competing tasks may none-the-less suffer CPU backlogs that lead to momentary and longer-lasting interruptions to the multimedia playback experience. As users look to manufacturers for playback devices that are less expensive and able to serve many other simultaneous applications, playback device performance will continue to limit the potential performance of multimedia applications.